1. Field of the Invention
The present invention relates to an active-type band-pass filter (referred to as "active-type BPF" hereinafter), and more particularly to an active-type BPF for use in a mobile communication system such as a portable telephone, mobile phone, and the like.
2. Description of the Prior Art
According to a conventional active-type BPF design, a no-load Q (Quality) factor of a practical resonator is around 1,000 in a transverse electromagnetic mode (referred to as "TEM mode" hereinafter), 6,000 in a transverse magnetic mode (referred to as "TM mode" hereinafter), and 20,000 in a transverse electric mode (referred to as "TE mode" hereinafter), where the no-load Q factors mentioned above are improved in the negative regions.
When a no-load Q factor of a resonator is designed to be, for example, five times greater than that of an original one, even if such a resonator is used as an active-type BPF, the resonator is accompanied by an excessively great noise figure (referred to as "NF" hereinafter). Therefore, it has been impossible to provide any practical active-type BPF.
The NF can be reduced when by increasing the no-load Q factor of the resonator. However, because the no-load Q factor of the resonator is approximately proportional to the size of the resonator, the resonator size is increased when the no-load Q factor of the resonator is increased. Thus, the resulting size of the resonator is too large to use the resonator as an active-type BPF.